1. Field of the Invention
The present invention relates generally to a solid electrolytic capacitor with a valve-action metal such as aluminum, tantalum, or the like used for an anode and an electroconductive polymer used for a solid electrolyte, and to a method of manufacturing the same.
2. Related Background Art
Conventionally, solid electrolytic capacitors have been used widely in computers, portable telephones, and the like.
FIG. 5 shows a configuration of a conventional solid electrolytic capacitor with a valve-action metal used as an anode. In FIG. 5, numeral 5 indicates a valve-action metal porous body, numeral 6 a dielectric oxide coating, numeral 7 a solid electrolyte layer, numeral 8 a carbon layer, numeral 9 a silver (Ag) paste layer, numeral 10 an anode leading-out terminal, and numeral 11 a cathode leading-out terminal. Initially, the dielectric oxide coating 6 is formed on a surface of the valve-action metal porous body 5 such as aluminum with a roughened surface, powder-sintered tantalum, or the like. Next, an electroconductive polymer such as polypyrrole, manganese dioxide, or the like is formed on the surface of the dielectric oxide coating 6 as the solid electrolyte layer 7. Subsequently, a cathode layer including the carbon layer 8, the silver paste layer 9, and the like is formed on the solid electrolyte layer 7. Thus, a capacitor element is produced. Afterward, the anode leading-out terminal 10 is attached to an anode lead part by welding or the like and the cathode leading-out terminal 11 is attached to the cathode layer with a conductive adhesive. Finally, a package (not shown in the figure) is formed to cover the whole capacitor element except for parts of the anode and cathode leading-out terminals. Thus, a solid electrolytic capacitor is obtained. The package serves for maintaining airtightness from the outside. Generally, the package is a tip type package formed with a mold using an epoxy-based thermosetting resin containing a silicon oxide filler or the like, or a lead wire type package formed by dipping. With this configuration, in order to obtain a lower equivalent series resistance (ESR), it is necessary to increase the conductivity of the solid electrolyte layer and to give consideration to characteristics of the materials of the carbon layer and the silver paste layer and methods of forming them.
There is a solid electrolytic capacitor with a configuration in which an anode body and a cathode body are laminated so that an increased capacitance of the product and a lower ESR are obtained (JP 11(1999)-219861 A). In JP 11(1999)-219861 A, a method of manufacturing the solid electrolytic capacitor is proposed in which a solid electrolyte layer is connected directly to a cathode leading-out electrode. In a conventional solid electrolytic capacitor with an electroconductive polymer used for a solid electrolyte, in order to form the electroconductive polymer inside pores uniformly by increasing reaction resistance in a polymerization reaction and in order to improve the adhesive strength between an electroconductive polymer layer and a polymerized body when the electroconductive polymer is formed using an electrolyte (a polymeric monomer solution) by electrolytic polymerization, generally a binder resin is contained in the electrolyte (a polymeric monomer solution) as proposed in JP 1949637 (1995). Usually, such a binder resin also is contained in the electroconductive polymer layer.
Since a binder resin is contained in the electroconductive polymer layer of a conventional solid electrolytic capacitor, there has been a problem in that the resistance of the electroconductive polymer increases. On the other hand, when the binder resin is omitted to reduce the resistance, the electroconductive polymer layer is peeled off easily. In addition, even when the binder resin is present, the electroconductive polymer may be peeled off partially from a polymerized body during a drying step.
When a solid electrolytic capacitor is formed using a metal foil for a cathode body with an anode body and the cathode body connected directly with each other through an electroconductive polymer formed on the anode body, it is necessary to apply a sufficiently high pressure to secure sufficient adhesion of the electroconductive polymer so as to increase the connection area. Even when a pressure of 100 kgf/cm2 is applied, an ESR of 10 mxcexa9 or lower cannot be obtained (for an element effective area of 3xc3x975 mm2). In this case, when the pressure to be applied is low, the area where the metal foil as the cathode body and the electroconductive polymer are in contact with each other also is reduced and thus the contact resistance tends to increase.
Hence, in order to increase the contact area to lower the ESR, the package must be formed so that a high pressure is applied to the contact surface. However, it is difficult to form such a package, and there is a possibility that the dielectric oxide coating may be broken by the pressure.
In order to solve the conventional problems described above, it is an object of the present invention to provide a solid electrolytic capacitor with small variations in characteristics, a lower ESR, and an excellent high frequency property and to provide a method of manufacturing the same.
In order to achieve the above-mentioned object, a solid electrolytic capacitor of the present invention includes an anode body formed of a valve-action metal with a dielectric oxide coating layer formed on its surface; a cathode body; and an electroconductive polymer layer disposed between the anode body and the cathode body. The electroconductive polymer layer contains a softener for softening the electroconductive polymer layer.
A method of manufacturing a solid electrolytic capacitor, which includes an anode body formed of a valve-action metal with a dielectric oxide coating layer formed on its surface, a cathode body, and an electroconductive polymer layer disposed between the anode body and the cathode body, of the present invention includes: forming an electroconductive polymer layer on at least one electrode body selected from the anode body and the cathode body; impregnating the electroconductive polymer layer with a softener diluted with a low-boiling-point solvent to soften the electroconductive polymer layer; and evaporating the low-boiling-point solvent.